1. Field of the Invention
The invention is directed to a process for treating polymer-containing workpiece surfaces and to an aqueous non-aging solution.
2. Description of the Prior Art
Plastics can be coated with metals by wet-chemical processes according to various methods. Conventionally, the polymer surfaces are hydrophilized and roughened and then rendered conductive so that metal can be electrodeposited. Until a few years ago, it was conventional to seed the prepared surface with a catalytically active metal for this purpose and then to deposit a metal, usually copper, by chemical reduction from a suitable bath. In the meantime, processes have been applied which allow metal plating directly without electroless baths. An electrically conducting layer is produced instead of the catalytically active activator. This layer may comprise metal, e.g., palladium, but generally comprises a metal chalcogenide, a carbon layer produced from a carbon suspension, or an electrically conducting polymer, e.g., polypyrrole.
The following polymers are used as plastics: epoxy resin, polycarbonate, poly(phenylene oxide), polyester, polyamide, polyimide, poly(ether imide), polystyrene, poly(vinyl chloride) or acrylonitrile-butadiene-styrene copolymers (ABS).
In the fabrication of printed circuit boards in electronics, double-sided printed circuit boards or multilayer circuits require that the walls of the drilled through-holes be coated with metal so as to make contact between the metallic layers and to join wired components by soldering. For this purpose the applied metal layer should adhere securely to the printed circuit board dielectric and to already existing metal, e.g., the metal lining, and the inner layers of multilayer circuits, and should withstand the soldering process in particular.
A strong bonding between the plastic substrate and the metallic cover layer is also necessary in decorative metallization of plastic articles. This is equally true for functional metallization of articles, e.g., for shielding against electromagnetic radiation.
The strong bonding between the deposited metal and the polymer substrate and accordingly the functional quality of the workpiece to be produced is determined chiefly by the type of preconditioning process and the materials employed in this process. Usually, the first process step consists in swelling the polymer matrix. The second process step consists of an oxidative etching step which degrades and accordingly roughens the polymer matrix. Acceptable etching media are alkaline permanganate solution or chromic acid. However, other processes are known in which pretreatment consists only of swelling the substrate and not treating it in an oxidative etching solution.
Micro-roughening of the polymer substrate is required particularly for the preconditioning of through-holes in printed circuit board material so that deposited metal can adhere well. In order to clean through-holes smeared with polymers when drilled, an etching medium such as alkaline permanganate solution can remove drilling dust and smeared polymer material; however, micro-roughening is only achieved when the printed circuit board polymer has been swollen with an appropriate swelling solution prior to the etching step.
Swelling solutions generally contain water, an organic swelling agent and, usually, a substance reacting alkaline in water. As a rule, the organic swelling agents employed have high boiling points and only limited miscibility with water. Aqueous solutions of ethylene glycol derivatives are usually used for swelling polymers.
U.S. Pat. No. 3,898,136 describes the surface treatment of articles of epoxy resin and poly(phenylene oxide) resin with solvents having the general formula RO(AO).sub.n R', where R or R' represent an alkyl group or acyl group with not more than four carbon atoms and the other group represents a hydrogen atom, an alkyl group or acyl group with not more than four carbon atoms, A represents a branched or unbranched C.sub.2 - to C.sub.4 -alkene group, and n represents a whole number from 1 to 4, followed by etching to achieve a strong bonding of the substrate and the subsequently applied metal layer.
In DE-A 37 40 369, the swelling agent is combined with an inorganic or organic base which promotes the swelling action by hydrolyzing polar chemical bonds of the polymer.
U.S. Pat. No. 4,775,557 describes the pretreatment of polycarbonates with compounds having the general/formula R.sub.1 (OCH.sub.n H.sub.2n).sub.m --R.sub.2, where R.sub.1 and R.sub.2 represent independently aryl groups or alkyl groups with 1 to 4 carbon atoms, n=2 -4 and m=1 -5, and the subsequent etching treatment to achieve good coverage of the polymer surface with metal to be deposited subsequently and a good bonding strength.
WO 89/10431 describes a swelling solution containing an organic solvent which is immiscible with water, where a homogeneous, clear solution is obtained by adding a surface-active substance. The disadvantage in using surface-active substances consists in that the swelling solutions containing these compounds generally foam considerably, which limits their applicability.
U.S. Pat. No. 5,049,230 describes a pretreatment process for polymers for subsequent etching in which mixtures of organic compounds which etch the polymer without attacking it are used in an alkaline medium. Organic compounds, e.g., lower alcohols such as methanol, ethanol, isopropanol, alkanolamines and phenolamines, tertiary amines, carboxylic acids, etc., are used to solubilize the components.
WO 85/05755 describes a swelling solution containing compounds having the general formula R.sub.1 O--C--(CH.sub.2).sub.m --C--OR.sub.2 and R.sub.3 --(OCH.sub.2 CH.sub.2).sub.n O--R.sub.4, where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 independently represent either hydrogen atoms, aryl groups or alkyl groups with 1 to 4 carbon atoms, m=0-2 and n=2-5. Preferred compounds are propylene glycol monomethyl ether and propylene glycol monopropyl ether or tetraethylene glycol in alkaline solution.
The processes and pretreatment solutions mentioned above have the disadvantage that at higher temperatures, particularly when accompanied by a high content of alkalizing agents such as alkali hydroxides, the swelling solution separates since, by reacting with the carbon dioxide contained in the air, the alkalizing agents change into carbonates which are poorly soluble in the swelling solution. The chemical reaction taking place is described by the following reaction equation: EQU CO.sub.2 +2 NaOH.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 O.
One of the liquid phases occurring in the separation of the solution mostly contains the swelling agent, while the other consists predominantly of water. The higher the operating temperature and the higher the concentration of alkali hydroxide in the swelling solution, the faster the formation of the carbonates promoting the separation of the swelling agent. Therefore, after a determined operating time, the solution must either be diluted with water and replenished with fresh solvent or it must be discarded completely.
Another known possibility for allaying these problems consists in liberating the swelling solution of its salt content or operating at a lower temperature so that separation only occurs after a longer period of operation. However, these measures are cumbersome, costly and harmful to the environment since valuable raw materials are consumed, the treatment installations are not available for treating the product during the required maintenance periods, and large amounts of raw materials which present disposal problems must be discarded.
The disadvantages mentioned above have been tolerated in the past, but it is not economical to work with the known swelling agents for the reasons given above, since these agents are not resistant to aging.